<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .slider-bar {
            position: absolute;
            left: 50%;
            top: 300px;
            margin-left: 600px;
            width: 45px;
            height: 130px;
            background-color: pink;
        }
        
        .w {
            width: 1200px;
            margin: 10px auto;
        }
        
        .header {
            height: 150px;
            background-color: purple;
        }
        
        .banner {
            height: 250px;
            background-color: skyblue;
        }
        
        .main {
            height: 1000px;
            background-color: yellowgreen;
        }
        
        span {
            display: none;
            position: absolute;
            bottom: 0;
        }
    </style>
</head>

<body>
    <div class="slider-bar">
        <span class="goBack">返回顶部</span>
    </div>
    <div class="header w">头部区域</div>
    <div class="banner w">banner区域</div>
    <div class="main w">主体部分</div>
</body>

<script>
    let sliderBar = document.querySelector('.slider-bar');
    let goBack = document.querySelector('.goBack');
    let banner = document.querySelector('.banner');
    let main = document.querySelector('.main');
    let bannerTop = banner.offsetTop;
    let sliderTop = sliderBar.offsetTop;
    let mainTop = main.offsetTop;
    document.addEventListener('scroll', function() {
        if (window.pageYOffset > bannerTop) {
            sliderBar.style.position = 'fixed';
            sliderBar.style.top = sliderTop - bannerTop + 'px';
        } else {
            sliderBar.style.position = 'absolute';
            sliderBar.style.top = '300px';
        }
        if (window.pageYOffset > mainTop) {
            goBack.style.display = 'block';
        } else {
            goBack.style.display = 'none';
        }
    });
</script>

</html>